One of the major processes used in the foundry industry for making metal parts is sand casting. In sand casting, disposable foundry shapes (usually characterized as molds and cores) are made by shaping and curing a foundry mix which is a mixture of sand and an organic or inorganic binder. The binder is used to strengthen the molds and cores.
The two major processes used in sand casting for making molds and cores are the (a) cold-box process and the (b) no-bake process. In the cold-box process, a gaseous curing agent is passed through a compacted shaped mix to produce a cured mold and/or core. In the no-bake process, a liquid curing catalyst is mixed with the sand and shaped into a core or and/or mold.
The major cold-box process is based upon polyurethane-forming binders. See for example U.S. Pat. Nos. 3,409,579 and 3,676,392. These systems are cured with a gaseous tertiary amine catalyst. The polyurethane-forming binder system usually consists of a phenolic resin component and polyisocyanate component which are mixed with sand prior to compacting and curing to form a foundry mix.
When the two components of the polyurethane-forming binder system are mixed with the sand to form a foundry mix, they may prematurely react prior to curing with the gaseous catalyst. If this reaction occurs, it will reduce the flowability of the foundry mix when it is used for making molds and cores, and the resulting molds and cores will have reduced strengths. This reduced flowability and decrease in strength with time is related to the benchlife of the foundry mix.
Sufficient benchlife of the foundry mix is important to the commercial success of these binders. Benchlife is the time interval between forming the foundry mix and the time when the foundry mix is no longer useful for making acceptable molds and cores. A measure of the usefulness of the foundry mix and the acceptability of the molds and cores prepared with the foundry mix is the tensile strength of the molds and cores. If a foundry mix is used after the benchlife has expired, the resulting molds and cores will have unacceptable tensile strengths.
Because it is not always possible to use the foundry mix immediately after mixing, it is desirable to prepare foundry mixes with an extended bench life. When polyurethane-forming cold-box binders are used, generally a compound which improves the bench life of the foundry mix must be added to the binder, usually the polyisocyanate component of the binder.
Among the compounds useful to extend the bench life of the foundry mix are organic and/or inorganic phosphorus containing compounds. Examples of organic phosphorus-containing compounds used as benchlife extenders with polyurethane-forming binder systems are disclosed in U.S. Pat. No. 4,436,881 which discloses certain organic phosphorus containing compounds such as dichloroarylphosphine, chlorodiarylphosphine, arylphosphinic dichloride, or diarylphosphinyl chloride, and U.S. Pat. No. 4,683,252 which discloses organohalophosphates such as mono-phenyldichlorophosphate.
Examples of inorganic phosphorus-containing compounds which extend the bench life of polyurethane-forming binder systems are disclosed in U.S. Pat. No. 4,540,724 which discloses inorganic phosphorus halides such as phosphorus oxychloride, phosphorus trichloride, and phosphorus pentachloride, and U.S. Pat. No. 4,602,069 which discloses inorganic phosphorus acids such as orthophosphoric acid, phosphoric acid, hypophosphoric acid, metaphosphoric acid, pyrophosphoric acid, and poly-phosphoric acid.
Carboxylic acids, such as citric acid, are also used to extend the benchlife of polyurethane-forming foundry binders. See U.S. Pat. No. 4,760,101.
As can be seen, there are numerous benchlife extenders for polyurethane-forming cold-box binders which reflects the interest in extending the benchlife of the foundry mix. Despite the cited work, there is still a need for amine-cured binder systems with longer benchlife.